DESCRIPTION: The overall objective of the research proposed in this application is to elucidate molecular mechanisms used to regulate pre-mRNA splicing during development of higher metazoan organism using a combination of molecular biology and biochemistry. The specific aims are as follows: 1. Refine biochemical reconstitution assays to study the somatic repression of P element IVS3 pre-mRNA splicing in vitro. 2. Identify and characterize the 65 kD and 40 kD RNA-binding proteins that are part of the P element IVS3 5' exon inhibitory complex. 3. Characterize the biochemical properties of the recombinant PSI (97 kD) and hrp48 proteins. 4. Analyze the expression patterns of PSI and isolate P element insertion mutations in the PSI gene; characterize mutations in the hrp48 gene.